Looking at vehicle_groups and the probability assigned each vehicle, I assume they are picked randomly out of the total of that group.
Of city vehicles:
2 out of 1000 cars on the road right now are using mini-nuclear reactors.
RVs: 3.34%
Swat Trucks : 1.34%
Road Roller: 1.34%
Armored car: 1.34%
Humvees (both types): 1.34%
APC (both types): 1%
Vehicles on the Highway:
No cars have mini reactors.
RVs: 6%
Swat Truck: 1.5%
Armored Car: 2.2%
Humvees (both types): 5.6%
APC and Advanced APC: 4.1%
Vehicles in Parking Lots:
4 out of 1000 cars in parking lots right now are using mini-nuclear reactors.
RVs: 1.64%
Swat Truck: 0%
Armored Car: 0%
Humvees (both types): 1.2%
APC and Advanced APC: 1.9%
Military vehicles should be removed from city spawn lists (maybe rare highway Humvees) and left to military roadblocks or expanded into military bunkers / bases with defenses. They have lots of ammo to remove and provide military armor shells. Luxury RVs are a lot more rare then military hardware, APCs are more common then RVs in parking lots. Swat trucks should be limited to police roadblocks and police stations IMO.
~2% of vehicles are armored cars, this is too high. Even ~4% of vehicles being RVs seems too high. And 2% of vehicles being meth labs seems like a bad neighborhood.